1. Field of the Invention
The invention relates to apparatus for scraping sludge from the surface of a liquid on which the sludge is floating, such as for example a sludge which is formed by flocculation of solid matter which is pushed up by flotation. We use the term "sludge" broadly to encompass scum or other floating solid matter.
2. Description of the Prior Art
In various techniques for water treatment a sludge mass is caused to float on the surface of a liquid and has to be removed. If the sludge contains valuable components, for example proteins from the waste water of slaughterhouses, it is important to remove the sludge as completely as possible, while at the same time it is desirable that the dry matter content of the sludge removed is as high as possible in order that the process of recovery is economical.
This kind of sludge is, in some known processes, obtained as a result of a flocculation and subsequent flotation treatment of the waste water. The protein-containing matter is, for example, first flocculated and then pushed up to the surface by means of air bubbles, so that a sludge mass is formed on the top of the fluid. It has to be noted, however, that the invention is not limited to this method of formation of a sludge mass; the layer of sludge can be formed in a different manner.
The techniques of flocculation and flotation are sufficiently well known in themselves that they do not need further explanation in this specification.
It has already been suggested that use should be made of the so-called stabilisation grid. This is a grid which is mounted at the surface of the liquid and which restricts the movement of the sludge at the surface. The purpose of this is to increase the content of dry matter in the upper layer of the sludge and to prevent too strong a stirring action in the sludge layer during scraping of the sludge from the liquid. Scrapers which move transversely over the grid are used in combination with the existing stabilisation grids.
There are objections to these existing systems. A significant part of the sludge layer which is already dry and which protrudes above the grid is pushed back below the grid by the scraper blades during the movement of the scraping mechanism. The stability of the sludge layer is disturbed by this downward motion. This is particularly disadvantageous for the following reason. When the sludge which already contains a high content of dry matter is pushed back under water, part of it, which has a specific gravity of more than one, again settles down in the liquid. After it has settled down this sludge is carried away with the effluent from the process, which firstly means a loss of a certain amount of the separated sludge and secondly increased pollution of the effluent. The sludge which has settled in this manner possesses a different structure from the freshly flocculated matter and because of this cannot be floated a second time.